Lead severing contrivance



April 1, 1958 W.W1DMAIER LEAD SEVERING CONTRIVANCE Filed March 2, 1955 m WCM/MPZ/MP INVENTOR. WimaM WIDMHIER BY Wflm Unite .1 Sr

LEAD SEVERING CONTRIVANCE Application March 2, 1955, Serial No. 491,609

4 Claims. (Cl. 313-102) This invention relates to severable lead-wire contrivances for use in inaccessible places such, for example, as the interior of an electron tube, and has special reference to the provision of improvements in the art of manufacturing electron tubes of the various kinds capable of utilizing such contrivances.

In the manufacture of electron tubes it is sometimes desirable to destroy the electrical continuity of one or more of its internal wires prior to operating the tube in its normal or intended manner. For example, it has previously been proposed (i) to evaporate photo-sensitive material onto an inner cathode surface of an evacuable envelope by means of an evaporator (e. g. a hot-wire) energized by current flowing through a conductor which is connected through a fuse to an electrode lead and (ii) to blow the fuse, subsequent to the evaporation process, by a surge of current applied through said lead.

The trouble with this procedure is that the fuse-blowing surge, however brief, passes through the evaporator and releases an additional or surplus quantity of vaporizable material. This problem has long been recognized and the usual practice is to halt the evaporation process before the desired ultimate quantity of vaporizable material has been laid down and to rely upon the fuse-blowing surge to finish the job. This, obviously, involves a guess on the part of the operator as to the exact additional quantity of vaporizable material to be supplied by the fuse-blowing surge. As a consequence, the response characteristics of photo-sensitive electrodes made in this manner may vary from tube to tube.

Accordingly, the principal object of the present invention is to obviate the above described and other less apparent objections to present day severable lead-wire installations.

The foregoing and related objects are achieved in ac cordance with the invention (a) by applying a vaporizing current to the vaporizable material through a conductor having a heat-severable portion, (b) halting the application of the current when the desired ultimate quantity of vaporized material has been deposited upon the appropriate surface and (c) subsequently, in the absence of current, severing the heat-severable portion of said conductor. The conductor is severed by subjecting the severable portion thereof to heat transferred thereto from an auxiliary source such, for example, as a radio frequency oscillator. The severable portion of the conductor comprises a fuse which is immune to the vaporizing current that flows therethrough in its transit to the vaporizer. The fuse is preferably biased to open, to ensure a clean break. When an activator, e. g. cesium, is used (to sensitize" the photo-sensitive material) or when a getter is used (to clean up residual gases within the tube), pellets of said materials may be supported on the same structure as the fuse, whereby they are in a position to be evaporated by the same auxiliary source of heat that is used to blow the fuse. The invention is described in greater detail in conneces Patent g tion with the accompanying single sheet of drawings, wherein:

Fig. l is an elevational view, partly in secton, of the invention as applied to a photo-multiplier; the tube being on the pump, and showing a photometer,-for gauging the thickness of the photo-cathode layer, and an R. F. oscillator for blowing the fuse in the severable lead that carries the vaporizing current to the vaporizable material of which said layer is comprised;

Fig. 2 is an enlarged view in perspective of the severable lead-wire contrivance used in the tube of Fig. l; and

Fig. 3 is a view, similar to Fig. 2, but showing a getter and cesium pellet as elements of the contrivance of the invention.

in Fig. 1 the invention is shown as applied to photomultiplier tube 1 of more or less standard form and dimensions. The tube 1 is shown on the pump and is at that stage of its manufacture whereat a photo-emissive material (e. g. antimony) in the form of a pellet 3 is being evaporated by means of a hot-wire 5 onto the inner surface 7 of the front end or window to form the cathode 9 of the device. The hot-Wire or resistance element 5 is strung between two supports 11 and 13, one of which is connected to the tubes disc-like accelerating electrode 15. The disc contains a central aperture (not shown) through which electrons normally pass in their transit to the first mutiplying electrode or dynode 17; hence the pellet 3 and the resistance ele ment 5 upon which it is supported should be ofi-set from said aperture. As is conventional, the dynodes l722, which are here shown in an in-line array, are supported adjacent to their side edges on a pair of insulating strips 23 and 25 and are each provided with a lead 1.8a, etc. which extends to the exterior of the envelope. A metal film 27 on the inner side wall of the envelope, and a resilient wire 29 which is biased to bear against said film, provide an electrical connection between the photoemissive cathode 9 and the cathode lead 9a.

in evaporating the cathodic material 3, two of the electrode leads, in this case the lead 15a to the accelerating electrode 15 and the lead 13a to one of the dynodes, comprise the circuit for supplying the heating or vaporizing current to the evaporator 5 from an external source, exemplified by the battery 31. One of these leads, e. g. the lead 18a, contains'the heat-severable contrivance of 1 the invention.

' quite rapidly by eddy currents induced therein from an external applicator such as the coil 39 in the output of a radio frequency oscillator 41.

The second element of the combination comprises a fuse 35 constituted of a material which is immune to the heating current which. passes therethrough during the evaporation process but which will melt or blow when subject to the much higher degree of heat to which it is subjected by reason of its direct contact with the heated loop or secondary 33 of the transformer 33 and 39.

. Aluminum is preferred as the fuse material because its or certain alloys thereof may be employed as the fuse material.

The third element of the contrivance comprises a bowed metal (e. g. tungsten) spring 37 which is connected at one end to the fuse strip 35 and firmly anchored at its other end, as by a weld to the insulated terminal 13 of the evaporator The spring 37 is not essential to the practice of the invention but when used exerts its biasing force on the fuse 35 and hence ensures a rapid and clean break in the series connection between the evaporator 5 and its current source when the fuse melts under the influence of heat supplied thereto through the loop or secondary 33 of the transformer 33, 39, by the oscillator 4-1.

When the vaporizable cathodic material 9 laid down on the foundation surface 7 by the evaporator 5 is of a type requiring the application of a film of sensitizing material, the cesium or other sensitizer may be supported on the same structure as the fuse, instead of being sup ported separately on the mica strip 25, as shown at 43, Fig. 1. Thus, as shown in Bi". 3 the sensitizing material for the cathodic layer 9 may be supported within a container which may itself form the secondary by virtue of its flattened, round form. The container i5 is adapted to be heated by the same auxiliary source of heat (4-1, Fig. 1) that is used to biow the fuse 35. When the walls of the container 4S are heated the heat is conducted to the sensitizing material therein and vaporizes said material. The container 45- has apertures 46 through which the vaporized sensitizing material may pass to thecathode 5* through the evacuated interior of the tube. in order to insure that all of the sensitizing material is vaporized when the container is heated, cesium may be introduced into the tube by means of an exothermically reactive mixture which releases cesium on activation. For example, a mixture in the form of a powder may be used having a composition, by weight, of about 2 parts of cesium chromate to about 1 part of silicon. When the mixture is heated to its activation temperature cesium is released in vapor form.

The cesium sensitizing material may be vaporized after the fuse is blown even though the sensitizing material has an activation temperature which is lower than the temperature at which the fuse blows. Induced radio frequency energy displays a phenomenon wherein the outer portions of a receiver of induced R. F. energy are heated more rapidly than the inner portions. Therefore, the outer surface of the container walls are heated more rapidly than the sensitizing material within the walls. Since more of the energy picked up by the heating memer is concentrated along the outer portions thereof as the frequency of the induced energy is increased, the use of a relatively high frequency, say about one megacycle per second, may be used to melt the fuse before the sensitizing material is evaporated.

The container 45 may also serve as a support for the getter used to clean up residual gas in the envelope 1 (Fig. 1). The getter material which is shown in Fig. 3 in the form of a short-wire 47, is attached, as by solder, directly to the container 45. Since the getter 47 is the last thing to be flashed its flash point should be higher than those of the fuse 35 and the sensitizing material in the container 45*. A one-to-one mixture, by weight, of barium and aluminum provides a getter material of a satisfactorily high flashing temperature, in this case, 1150 C. Another example of a high-temperature getter, in this case of the chemical reaction type, comprises about 100 parts of barium titanate to about 3.75 parts of beryllium. Its flashing temperature is about 1450 C.

Operation When the switch 69 in the circuit that supplied'heating current to the resistance element 5 is closed the cathodic material 3 is heated and gradually vaporizes. The vaporized material condenses and forms a film or thin layer 9 upon the inner surface 7 of the tube window. The thickness and hence the response characteristic of this layer 9 may be gauged throughout its manufacture by a photometer device 51, including a phototube 53, actuated by light, from a lamp 55, passing through said layer. When the desired ultimate quantity of vaporized material has been deposited upon the window 7 the flow of current to the resistance element 5 is halted by opening the switch 49, either manually or automatically. Thereafter, in the absence of current in the resistance element 5, the fuse is blown by heat transferred thereto from the inductively heated disc or loop 33. Since there is no heating current flowing in the resistance element 5 at the time the fuse is being blown, the thickness and response-characteristics of the cathode layer are in no way affected by the fuseblowing step. Hence the operator is relieved of all guess work.

While the invention has been described as applied to the manufacture of a phototube having a cathode of the photoemissive variety it is to be understood that the disclosure in this respect is illustrative, and that the invention is likewise applicable to other types of photoresponsive tubes having different varieties of photosensitive elements. For example, the invention may be applied to the manufacture of image-orthicons, vidicons and other varieties of pick-up tubes.

What is claimed is:

l. A severable lead-wire contrivance for use in an electron tube as a series connection between an electrode lead and a resistance element, said contrivance comprising a metallic element adapted to be heated by eddy currents induced therein from an external source, a fuse strip in said tube and connected at one end to said heater element and adapted to be blown solely by heat transferred thereto by conduction from said element, and a metal spring connected to the other end of said fuse strip and biased to open the same.

2. An electron tube comprising an envelope, a plurality of electrodes within said envelope, an electrical terminal connected to each of said electrodes within said envelope and extending through a wall of said envelope, an electrical circuit coupled between two of said electrodes and within said envelope, a vaporizable substance in heat transfer relation with a portion of said circuit, said circuit being adapted for heating said substance by means of electric current flow in said entire circuit, a fuse in said circuit and in said envelope and physically removed from said portion; and means other than said current flow for heating and blowing said fuse.

3. An evacuated envelope containing a surface to be sensitized, a resistance element mounted adjacent to said surface, a sensitizing material for said surface supported on said resistance element and adapted to be vaporized by heat generated as an incident to the flow of current through said element, a series circuit connected to said resistance element and extending to the exterior of said envelope, said series circuit comprising (1) a fuse mounted in said envelope and adapted to pass an electric current suificient to subject said sensitizing material to its vaporizing temperature, and (2) an inductive element mounted in said envelope and in heat transfer relationship with respect to said fuse and adapted to be heated to a temperature sufiicient to blow said fuse, in the absence of said heating current, by currents induced therein from an external source.

4. A severable electrical circuit within an envelope comprising an inductive element adapted to be heated by induction, a fusable member in physical contact with said inductive element and of a size capable of being fused only by induction, a resistance element in said circuit, a vaporizable material mounted on said resistance element, said resistance element being adapted to be heated to a 6 temperature suificient to vaporize said material solely by 538,284 Rolfe Apr. 30, 1895 current How in said circuit. 1,122,478 Cole Dec. 29, 1914 References Cited in the file of this patent gfi 1 83:33 UNITED STATES PATENTS 5 2,676,282 Polkosky Apr. 20, 1954 Re.21,326 Rava Jan. 16, 1940 

